The present invention relates to a temperature-controlled crystal oscillator circuit having particular, but not exclusive application, in master oscillator and/or local oscillator circuits in transportable radio communications equipments, for example mobile and portable transceivers and paging receivers.
With the advent of narrow radio channels in recent years, there is a need for stable oscillator circuits particularly in transportable equipments which may have to operate over a typical temperature range of -30 degrees C. to +70 degrees C. Crystals, particularly AT-cut quartz crystals, are used as a frequency stabilizing element. However it is well known that AT-cut quartz crystals drift in response to temperature changes. One method of countering the problem of drift is to place the crystal in a temperature-controlled oven. However such ovens consume power, which is undesirable for battery-powered transportable equipment.
Another method is to generate a correction voltage which is applies to a frequency pulling element, such as a varicap diode, in an oscillator circuit.
In one example of the method in which a correction or compensating voltage is produced, disclosed in SU 1136299A, compensating signal zones are formed on an oscillator temperature-frequency characteristic of say an oscillator having as AT-cut crystal. These zones lie on either side of the point of inflection lying on the abscissa, which point is referenced t.sub.2 in the single figure of SU 1136299A. The first of the zones lies between the maximum in the characteristic which occurs at a temperature t.sub.1 (which is less than t.sub.2) and the second zone lies between t.sub.2 and the minimum in the characteristic which occurs at a temperature t.sub.1 (which is less than t.sub.2 and the second zone lies signal for each zone, that is zones (t.sub.2 -t.sub.1) and (t.sub.3 -t.sub.2) comprises a signal which varies in accordance with an exponential function. The start and end of the compensating signal waveform for the first zone is determined by the maximum point and the point of inflection and for the second zone by the point of inflection and the minimum point. Once the characteristic has been generated, it is shifted so that at a particular temperature, the maximum point corresponds to the rated frequency. SU 1136299A does not illustrate a circuit by which this method is implemented.
A voltage generating circuit for producing a correction voltage is disclosed in European Patent Specification 0129618. This known circuit comprises a temperature sensor whose output is connected to a drive frequency pulling control element via a power series function generator and a summing amplifier. The temperature sensor is adapted to provide an electrical output f(t) that is a linear varying function of temperture. The summing amplifier is adapted to provide a weighted sum of the power series function generator outputs, which sum is applied to the control element. The power series function generator is adapted to generate a series of Chebyshev-like outputs of which the nth output is a polynomial function in f(t) of order (n-1). The summing amplifier sums four or more different outputs of the power series function generator. The power series may be derived using the Weierstrass theorem in which: EQU V(T)=A.sub.0 +A.sub.1 (T-T.sub.0)+A.sub.2 (T-T.sub.0).sup.2 +A.sub.3 (T-T.sub.0).sup.3 +. . . A.sub.n (T-T.sub.0).sup.n
where
V(T) is the required compensating voltage PA0 T is the working temperature PA0 T.sub.0 is the inflection temperature, and PA0 A.sub.0, A.sub.1, A.sub.2 etc. are the summing coefficients. PA0 V.sub.comp is the compensating voltage, PA0 T.sub.R is a reference temperature in degrees Kelvin, PA0 T is the working temperature in degrees Kelvin, and PA0 a1, a2, b and c are constants.
Also known is U.S. Pat. No. 3,821,665 which discloses a circuit for producing a correction voltage in accordance with a power series comprising three or more terms.
Although basing temperature compensation on generating a power series function will give acceptable results there is always room to make improvements.